How USB transmits data

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Quick Questions

Shade in the A or B circle for each question.

1. How does a USB cable send data?

A Serially, one bit at a time down a single wire

B In parallel, with many bits sent at once side by side

2. Which statement about the direction of USB data is correct?

A Data can travel in both directions along a USB connection

B Data can only travel from the device to the computer

3. Older versions of USB are described as which type of transmission?

A Simplex: one direction only, all of the time

B Half-duplex: both directions, but only one at a time

4. Later versions of USB are described as which type of transmission?

A Half-duplex: both directions, but never at the same time

B Full-duplex: both directions at the same time

5. Which USB statement about power is true?

A A USB cable can carry power as well as data

B A USB cable can only carry data, never power

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1USB: bit, bit, bit, bit ->		AUSB is not simplex. It carries data in both directions: half-duplex in older versions, full-duplex in later ones.
2USB 1.x / 2.0: send OR receive		BUSB 3.0: send <-> receive at once
3Later USB versions use full-duplex, so data can travel in both directions at the same time		COlder USB versions use half-duplex, so data travels in both directions but only one way at a time
4The USB cable carries power as well as data, so a simple device needs no separate power supply		DUSB is serial, not parallel. It sends one bit at a time down the cable.
5USB sends 8 bits at once on 8 wires (parallel)		EUSB cable = data lines + power line
6USB is simplex: data goes one way only		FUSB sends data serially, one bit at a time, down the cable

Definitions

USB serial transmission

Fill in the Blanks

Complete each line of code by filling in the missing parts.

usb.transmission ← ______

usb.bitsAtATime ← _

IF usbVersion = 1 THEN mode ← ___________ ENDIF

IF usbVersion = 3 THEN mode ← ___________ ENDIF

cable.carries ← [data, _____]

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A OUTPUT "USB sends data"

B OUTPUT "one bit at a time"

C OUTPUT "along a single cable"

D OUTPUT "so it is parallel"

Question 2

A mode ← "serial"

B bitsPerTick ← 8

C power ← TRUE

Question 3

A version ← "USB 1.1"

B duplex ← "full"

C OUTPUT "one way at a time"

Question 4

A version ← "USB 3.0"

B duplex ← "full"

C OUTPUT "one way at a time"

Question 5

A bits ← "10110001"

B FOR i ← 1 TO 8

C SEND bits ALL AT ONCE

D NEXT i

Answer Key

How USB transmits data

Section A: Quick Questions

A — Serially, one bit at a time down a single wire

A — Data can travel in both directions along a USB connection

B — Half-duplex: both directions, but only one at a time

B — Full-duplex: both directions at the same time

A — A USB cable can carry power as well as data

Section B: Match the Pairs

USB: bit, bit, bit, bit -> → USB sends data serially, one bit at a time, down the cable

USB 1.x / 2.0: send OR receive → Older USB versions use half-duplex, so data travels in both directions but only one way at a time

Later USB versions use full-duplex, so data can travel in both directions at the same time → USB 3.0: send <-> receive at once

The USB cable carries power as well as data, so a simple device needs no separate power supply → USB cable = data lines + power line

USB sends 8 bits at once on 8 wires (parallel) → USB is serial, not parallel. It sends one bit at a time down the cable.

USB is simplex: data goes one way only → USB is not simplex. It carries data in both directions: half-duplex in older versions, full-duplex in later ones.

Section C: Definitions

USB serial transmission — USB sends data serially, one bit at a time, down a single cable. It works in both directions: half-duplex in older versions, full-duplex in later ones. The cable can also carry power.

Section D: Fill in the Blanks

usb.transmission ← serial

usb.bitsAtATime ← 1

IF usbVersion = 1 THEN
  mode ← half_duplex
ENDIF

IF usbVersion = 3 THEN
  mode ← full_duplex
ENDIF

cable.carries ← [data, power]

Section E: Spot the Error

D — USB sends one bit at a time down a single cable, so it is serial transmission, not parallel. The line should be OUTPUT "so it is serial".

B — Serial transmission sends only one bit at a time, so bitsPerTick should be 1. Sending 8 bits at once would be parallel.

B — Older versions of USB are half-duplex: data can travel in both directions, but only one direction at a time. duplex should be "half".

C — Later versions of USB are full-duplex, so data can travel in both directions at the same time. The output should be "both ways at once".

C — USB is serial, so each bit is sent separately, one after another, along the cable. The loop should send bits[i] ALONG cable, not all the bits at once.